Low cost monolithic megahertz rate A/D converters are critical building blocks for many digital signal processing applications. There are generally two types of A/D converters: the single-clock-cycle flash type and the multiple-clock-cycle subranging type. Both of these types include basically the same types of components, which include reference voltage ladders, comparators and apparatus for combining the outputs of the comparators into a digital signal. The major difference is that flash type A/D converters require a comparator for each division of the reference voltage ladder, i.e. each division representing a most significant bit (MSB) and each division representing a least significant bit (LSB) between each of the MSBs. Whereas, subranging type A/D converters only require a comparator for each MSB and a comparator for each LSB between approximately a single pair of MSBs. A plurality of electronic switches are provided to switch the LSB comparators to the portion of the reference voltage ladder which most nearly compares to the unknown voltage being digitized. This substantial reduction in the number of comparators greatly reduces the chip size.
There are several problems in the subranging type of A/D converters, including of course the fact that they require multiple clock cycles, which reduces them to a fraction of the speed of the flash type A/D converter. Also, because of the electronic switches and excess wiring required, switching and other noise is introduced into the signal channel, which causes some reduction in the accuracy. Much of this switching noise is introduced into the reference voltage ladder by each switching line crossing the reference voltage ladder. Further, the LSB comparators utilized do not cover certain steps, or areas, of the MSBs so that errors can occur at particular voltage levels.